A fiber optic gyroscope as adapted to measure a rotational angular velocity (angular velocity) of an object by using the Sagnac effect is widely used, particularly among the aircraft and rocket industries. The conventional fiber optic gyroscope is generally called an interference type fiber optic gyroscope which includes a light source to emit a light from one end surface, an optical fiber coupler to divide the light from the light source into two lights, and an optical fiber loop to allow the two divided lights to circulate therethrough in the respective opposite directions, wherein an angular velocity is measured based on variation of a phase (phase difference) attributed to the Sagnac effect between the two lights circulating in the optical fiber loop in the respective opposite directions (for example, Japanese Patent Application Laid-Open No. H6-74775).
Meanwhile, recently, an optical gyroscope (hereinafter described as a diode ring laser gyroscope as appropriate) is proposed in which a laser diode for emitting lights from both end surfaces thereof is disposed in an optical fiber loop, whereby the laser diode and the optical fiber loop in combination form a laser resonant circuit (laser resonator) (for example, Japanese Patent Application Laid-Open No. 2007-71614).
FIG. 3 schematically shows the composition of an optical gyroscope 50 as the diode ring laser gyroscope disclosed in Japanese Patent Application Laid-Open No. 2007-71614. The optical gyroscope 50 includes a semiconductor optical amplifier (SOA) 51 and an optical fiber 52 which is looped and which has its both ends connected to end surfaces 51A and 51B of the semiconductor optical amplifier 51, respectively. The semiconductor optical amplifier 51 emits laser beams CW and CCW respectively from the both end surfaces 51A and 51B, also amplifies by stimulated emission the laser beams CW and CCW which have circulated through the optical fiber 52 in the respective opposite directions, and then emits the amplified laser beams CW and CCW into the optical fiber 52 again.
Parts of the laser beams CW and CCW propagating in the optical fiber 52 in the counter directions are introduced into an optical fiber 54 via an optical coupler 53, and then, overlapped on each other at an optical coupler 55. The laser beams CW and CCW overlapped on each other are introduced into an optical detector 57 via an optical fiber 56. The optical detector 57 performs square law detection of the overlapped laser beams CW and CCW, and thereby detects a beat signal attributed to a difference in oscillation frequency between the laser beams CW and CCW. The difference in the oscillation frequency between the laser beams CW and CCW is caused because the optical fiber 52 and the semiconductor optical amplifier 51 constitute the laser resonator together. That is, the laser resonator lengths of the laser beams CW and CCW experience an effective change due to the Sagnac effect which is attributed to the rotation of the table 60.
The beat signal detected by the optical detector 57 is input to a spectrum analyzer 58, whereby a beat frequency fB is detected. An angular velocity of the rotating object (optical fiber) is calculated by a detector 59 based on the following formula showing the relation between the beat frequency fB detected and an angular velocity Ω: fB=(4A/nλP)Ω where A represents an area of a region enclosed by the optical fiber 52, n represents a refractive index of the optical fiber 52, λ represents wavelengths of the laser beams CW and CCW, P represents a path length of the laser beams CW and CCW, and Ω represents an angular velocity.
Because the diode ring laser gyroscope described above detects the angular velocity of the object by a beat frequency, the diode ring laser gyroscope is essentially enabled to measure the angular velocity with high detection sensitivity. However, when the inventor of the present invention considered to reduce the size of an optical fiber loop for the purpose of downsizing the diode laser gyroscope, it turned out that it is hard or impossible to detect the angular velocity when the optical fiber loop has a diameter of, for example, about 100 mm (so-called palm-size).
Also, the diode ring laser gyroscope can possibly fail to detect an angular velocity precisely if a light amount from the light source decreases due to, for example, the product life cycle. In order to cope with this problem, another fiber optic gyroscope structured identically may be provided for backup purpose, but the cost and volume of the detection apparatus are increased almost double.
Moreover, if the angular velocity is small, the diode ring laser gyroscope fails to successfully detect the angular velocity due to so-called lock-in phenomenon. It is well known that in order to be free from the lock-in phenomenon, a mechanical dither system is provided for the diode ring laser gyroscope thereby mechanically vibrating the optical fiber loop. However, to mechanically vibrate the optical fiber can possibly result in increasing a noise component of a detection signal. Also, the provision of the mechanical dither system requires an additional space.